Coupled transmission lines are experimentally characterized by using 4-port S-parameter measurements in a broad frequency band (up to 20 GHz). Test patterns are designed and fabricated by using a ball grid array (BGA) package process. Symmetrically coupled transmission lines are decoupled into two eigen modes that can be readily determined from the measured S-parameters. Then transmission line parameters and signal transient waveforms are directly determined by using the measured S-parameters. It is shown that not only are the transmission line parameters frequency-dependent, but also the frequency-variant effects and nonideal characteristics of transmission lines have a substantial effect on signal transients and crosstalk noises.